Difference between association and dependency?

问题:

In a UML class diagram, what is the difference between an association relationship and a dependency relationship?

From what I know, an association is a stronger relationship than a dependency, but I'm not sure how it is stronger.

Any example would be more than welcome :)

回答1:

What is the difference between dependency and association?:

In general, you use an association to represent something like a field in a class. The link is always there, in that you can always ask an order for its customer. It need not actually be a field, if you are modeling from a more interface perspective, it can just indicate the presence of a method that will return the order's customer.

To quote from the 3rd edition of UML Distilled (now just out) "a dependency exists between two elements if changes to the definition of one element (the supplier) may cause changes to the other (the client)". This is a very vague and general relationship, which is why the UML has a host of stereotypes for different forms of dependency. In code terms, such things as naming a parameter type and creating an object in a temporary variable imply a dependency.

...

回答2:

An association almost always implies that one object has the other object as a field/property/attribute (terminology differs). A dependency typically (but not always) implies that an object accepts another object as a method parameter, instantiates, or uses another object. A dependency is very much implied by an association.

回答3:

In OOP terms:

Association --> A has-a C object (as a member variable)

Dependency --> A references B (as a method parameter or return type)

public class A {
    private C c;
    public void myMethod(B b) {
        b.callMethod();
    }
}

There is also a more detailed answer.

回答4:

Dependency is like when you define a method that takes a String(in Java, C#, as string is a object in them) as a parameter, then your class is dependent on String class.

Association is like when you declare a string as an attribute in your class. then your code is associated with the string class.

String name = null //: is a association.

回答5:

Dependency - A change in a class affects the change in it's dependent class. Example- Circle is dependent on Shape (an interface). If you change Shape , it affects Circle too. So, Circle has a dependency on Shape.

Association- means there is a certain relationship between 2 objects

(one-one, one-many,many-many)

Association is of 2 types-

1. Composition

2. Aggregation

   1) Composition- stronger Association or relationship between 2 objects. You are creating an object of a class B inside another class A

 public class A {
       B b;
       public void setB(){
         this.b= new B();
        }
     }

If we delete class A , B won't exist( B object is created inside A only).

Another example -Body & Liver .Liver can't exist outside Body.

2) Aggregation - weaker type of Association between 2 objects.

public class A {       
             B b;
             public void setB(B b_ref){
                 this.b= b_ref;   
                /* object B is passed as an argument of a method */
              }
   }

Even if you delete class A, B will exist outside(B is created outside and passed to Class A)

Another example of this- Man & Car . Man has a Car but Man & Car exist independently.

回答6:

Here: "Association vs. Dependency vs. Aggregation vs. Composition", you have a great vade mecum with uml class diagrams and code snippets. The author gives us a list of relationships: Association, Dependency, Aggregation, Composition in one place.

回答7:

A dependency is very general and lowering complexity is about diminishing dependencies as much as possible.

An association is a strong (static) dependency. Aggregation and Composition are even stronger.